1. Field of the Invention
This invention relates to sheet metal casing for multistage pump and its manufacturing method, and particularly to a sheet metal casing for multistage submerged pump that has simple structure to facilitate fabrication and assembly at low cost, and has improved sealing and pumping efficiency.
2. Description of the Prior Art
Conventional multistage centrifugal pumps (such as submerged pumps) usually includes a plurality of casings with passages and impellers housed therein and mounted serially on a rotationable spindle for pumping fluid by adding pressure. The multistage pump made by casting in the past is heavy and bulky and has lower pump efficiency. It is generally replaced by sheet metal casing pump nowadays.
In the design of multistage sheet metal casing pump, the main considerations include: manufacturing and assembly cost, pump efficiency and passage design, pressure resistance, leakage prevention, etc. These factors are rarely totally met by conventional sheet metal casing multistage pump. For instance, European Pat. Application No. 81110541.0 discloses a sheet metal casing structure for multistage pump shown in FIG. 1. It has a casing 11 made by punching and a partition member 12 to form a stage 2 for housing an impeller 3 and baffles 20 therein. The casing 11 has a curved portion 19 for engaging a front stage with a rear stage. At the juncture of the engagement, a seal ring 14 is provided. The dimension and precision of the curved portion 19 is difficult to control. When more than one stage being fastened axially, the positioning of the curved portion 19 is even more difficult and is prone to deform. The seal ring 14 tends to malfunction and result in leakage under pressure or vibration. Moreover there is no smooth passage inside the casing 11 and the impeller 13. It is easy to produce turbulence around the front end 112 during pumping operation and result in lower pump efficiency. In short, it has the drawbacks of poor axial positioning, easy leaking and low pump efficiency.
FIG. 2 illustrates another prior art disclosed in U.S. Pat. No. 5,234,317. It has a pump casing 21 made by punching and pressing. The pump casing 21 has an end rim 211 formed in a U-shaped member for engaging securely with a next stage pump casing. However it has a smaller contact area (sealing surface). The end rim 211 is prone to deform and open outwardly under strong pumping pressure or vibration. Leaking is still not avoidable. Furthermore the pump casing 21 cannot be made by merely punching operation. It needs pressing operation to finish all the fabrication work required. The cost is higher. And there is also no smooth passage inside the casing and may result in turbulence and lower pump efficiency. When two stage pumps are connected, the connection portion forms an S-shaped structure. While it helps to prevent deformation and leakage, it increases production cost.
FIG. 3 shows a still another technique seen in the market place. The pump casing 4 includes a front outer shell 41, a rear outer shell 42, a front inner shell 43, a rear inner shell 44, a hub 46, a front seal ring 47 and an impeller (not shown in the figure). The front outer shell 41 includes a front side ring 413, an axial positioning ring 411 and a radial positioning ring 412. The rear outer shell 42 includes an axial positioning end 421 and a radial positioning end 422 mating respectively with the rings 411 and 412. The rear inner shell 44 has a positioning end 441 at the rear end.
For assembly of a single casing 4, the vane 45 is firstly soldered to the front inner shell 43. Then the front outer shell 41, rear outer shell 42, front inner shell 43 and rear inner shell 44 are assembled together (otherwise the inner shells 43 and 44 cannot be put into the outer shells 41 and 42). Then circular soldering is made on the solder spots 48 around the outer shell.
For assembly of a multistage pump casing 4, the positioning end 44 of a front casing 4 will be made contact with the front side ring 413 of a rear casing. The axial and radial positioning end 421 and 422 of the front casing will be made contact with the axial and radial positioning rings 411 and 412 which may couple with a seal ring to prevent leakage. The front and rear inner shells 43 and 44 have curved inside surface to smooth flow passage and enhance pump efficiency.
However there are still disadvantages in this technique, notably:
1. It has too many components and costs too much to produce. Eight components are required, including the front and rear outer shells 41 and 42 front and rear inner shells 43 and 44, vane 45, hub 46, front seal ring 47 and the impeller. The molding cost and assembly time increase greatly. The hub 46 and front seal ring 47 also add to the cost.
2. It needs secondary machining that increases cost and production time. The axial and radial positioning ring 411 and 412, and the axial and radial positioning end 421 and 422 need high precision machining work to get the accurate dimension for making the required connection. An usual punching operation cannot produce that kind of precision. The machining also unavoidably reduces shell thickness and weaken the structural strength.
3. The circular soldering of the solder spots 48 are much more expensive and time-consuming than conventional spot soldering. It also tends to produce notsightly casing and lower dimension accuracy. Positioning for assembly is more difficult. The soldering property and strength is difficult to control. A secondary machining is needed after the soldering and may reduce casing thickness.
The prior arts set forth above thus cannot totally meet all the design factors such as production and assembly costs, pump efficiency and passage streamline, pressure resistance, leak control and prevention, etc. There are still a lot of room for improvement.